Studies of the biological potency of CBER reference grass preparations were carried out. Biologically defined Allergy Units were assigned to reference preparations of the following grasses: Bermuda, June, Meadow fescue, Orchard, Perennial rye, Redtop, Sweet vernal, and Timothy. Candidate reference preparations of Box elder and White oak were also assayed for assignment of biological potency units. A candidate latex preparation was studied in a small number of lates allergic subjects. There is insufficient data to determine whether its biological potency differs from the other reference preparations assayed.